(1) Field of the Invention
The present invention relates to a synchronizer, and in particular, to a digital audio synchronizer for compensating an audio signal for a time difference between the audio signal and an associated video signal whose timing is disturbed by being synchronized video signal to a reference video signal.
(2) Description of the Prior Art
In a multi-station TV broadcasting system, a video signal from a local station is frame-synchronized to a reference signal of the central station. The frame synchronization causes a time delay in the video signal, so that a time difference results between the synchronized video signal and an audio signal accompanying for this video signal. In order to compensate the time difference, a digital audio synchronizer or a digital audio-delay device is used.
In a known digital audio synchronizing system, the audio signal is digitized and then stored in a memory. The stored signal is then read out from the memory. Time-wise, the read-out address is offset from the write-in address by a number of address locations which produce a time difference equivalent to to the time difference between the reference signal and the synchronized video signal. Therefore, the output audio signal is delayed from the incoming audio signal by the same time difference as exists between the reference signal and the incoming video signal. Accordingly, the audio signal is synchronized to the video signal which in turn is synchronized to the reference signal.
In the known audio synchronizer, any variation or change of the time difference between the incoming video signal and the reference signal results in a jump in the read-out address. This produces a discontinuity in the output audio signal and an ensuing unacceptable click sound.
In order to resolve this problem, a method is known in the prior art wherein an audio data signal is repeatedly read-out from the same address in the memory or is read-out from a narrow range of addresses during the transition, the addresses being near the time-difference variations. But the discontinuity of the audio signal waveform is still unsatisfactorily resolved by this known method.
Another known method is to change the rate or frequency at which the audio signal is read from the memory. Although the discontinuity is not caused in the method, it takes, an undesirably, long time until the audio signal is delayed by the time-difference variation.